1. Via serial console

With a serial connection to your WRT, you don't have to use the ping bug or change your Linksys firmware. You can set boot_wait from the console, using the commands

#nvram set boot_wait=on
#nvram get boot_wait           (just to confirm, should respond with "on")
#nvram commit                  (takes a few seconds to complete)

You can also set boot_wait from the CFE boot loader (to enter CFE, reboot the router with "# reboot" while hitting CTRL-C continously)

CFE> nvram set boot_wait=on
CFE> nvram get boot_wait       (just to confirm, should respond with "on")
CFE> nvram commit              (takes a few seconds to complete)

Or via PMON (older models, hit CTRL-C repeatedly during powerup or reboot)

PMON> set boot_wait on
PMON> set nvram boot_wait